CIVL375 Experiment Settling in Secondary Clarifiers (Type III and IV) Introduction: Flow entering the secondary clarifier (after aeration) in a wastewater treatment plant consists of large concentration of particles Settling of these particles occurs at two stages First, particles will settle in what is referred to as zone or hindered settling (Type III) Second, particles accumulating at the bottom will undergo a compression settling (Type IV) where the particles are too close that settling occurs by compaction The main objective of this experiment is to design a secondary clarifier for a wastewater treatment plant and determine the sludge volume index (SVI) SVI is defined as the volume (in ml) occupied by gram of activated sludge after 30 minutes of settling SVI is used by treatment plant operators to regulate sludge pumping rate from the secondary clarifier A low SVI (i.e., 35-150) is supposed to indicate a good settling sludge and, therefore, a low return sludge pumping rate SVI is computed as: SVI = Volume of settled sludge after 30 (ml ) × 1000(mg / g ) suspended solids (mg ) in − liter cylinder To design the surface area of a secondary settling tank, two areas need to be determined: (1) the area required for clarification (Ac) This area is determined from the slope of the settling curve at early settling time such that Ac = Q vo where Q is the flow rate and vo is the overflow rate (i.e slope of the line of the settling curve at early settling times) (2) the area required for thickening (At) This area is determined by first finding tu from the settling curve using the procedure outlined in the curve below Note that to find tu you need to know the underflow design concentration (Cu) of the thickened sludge Cu usually ranges from 10,000- 20,000 mg/l By knowing Cu then Co Ho=Cu tu Co is the initial suspended solids concentration and Ho is the height of the cylinder Now the area needed for thickening can be determined: Q= At Ho/tu Sample Identification: Wastewater entering the secondary clarifier at Zakher Wastewater Treatment Plant Procedure: Mix the sample container well and take a sample to determine the suspended solids concentration (Co) using the filtration method Take duplicate or triplicate samples and then average Bring a 1-L graduated cylinder and measure the depth in cm to the 1000 ml mark Also, measure the internal diameter of the cylinder Bring a stop watch Mix the sample container well and pour in liter of sample in the cylinder immediately Start timing Monitor the interface as it settles and record the depth and time Requirements: Fill out the provided experiment data sheet and carry out the required calculations If you are requested to submit a report of this experiment then your report should contain (a) an introduction that ends with the objective (b) methodology including sample identification (c) results and discussion (d) conclusion (e) references and (f) an appendix that contains the filled data sheet CIVL 375: Experiment Data Sheet Settling in a Secondary Clarifier (Type III and IV) Attending Group Members Name ID Experimental Conditions Experiment date Sample collected from Sampling date Graduated cylinder height, cm Graduated cylinder diameter, cm Initial suspended solids, mg/l Trial Initial suspended solids, mg/l Trial Initial suspended solids, mg/l Trial Average suspend solids, mg/l Sludge Interface Height Time Height (min) (cm) Time (min) Height (cm) Time (min) Height (cm) Required Calculations Determine the SVI Plot the settling curve and determine the surface area of the settling tank assuming the flow rate is 1000 m3/day and the underflow concentration (Cu) is 15,000 mg/l